Ascertaining a charge state of a battery system that includes at least one battery cell and one additional energy store that is connected in series with the at least one battery cell and can be switched in and out. One method includes switching in the additional energy store; measuring a voltage of the additional energy store at predefined time intervals; ascertaining a capacity change of the additional energy store with reference to the measured voltage and to a characteristic voltage curve of the additional energy store; ascertaining a charge throughput of the additional energy store with reference to the ascertained capacity change of the additional energy store; and ascertaining the charge state of the battery system with reference to the ascertained charge throughput.